Agilmore, we use dipsticks that we make from doweling to measure the fuel in our tanks. If you know the capacity of the tank to "almost full", you can calculate how much diesel 1/4, 1/2 ,3/4, and full are. First put in 1/4, then make a mark with extra fine tipped pen with indelible ink around the dowel. Continue to add another 1/4, now you have 1/2, and mark the stick again. Continue in this way. Do this at the beginning of the season, when you want to fill the tank anyway. And now you will have a dipstick that you can use in the future. We store ours in the lazarette after use, and the fumes on it do not come into the living areas of the boat.

We have interior fills for our watertanks (boat came that way), and have also made floating dipsticks for them, again using the measured amounts method. I quite like it, because there's no "sight glass" possibility, as they are integral with the hull, underneath the cabin sole, and I can still check the filling, and avoid overfilling.

If the fuel tank is so located that you can practically drain the water off it into a container, that's great. Wouldn't apply to our keel tank, which is constantly submerged.